Self-sharpening cutting tool with hard coating

ABSTRACT

There is disclosed a cutting tool having a blade coated on one side with a hard coating living a laminar or layered microstructure. The coating tends to wear evenly and smoothly, thereby keeping a cutting edge of the cutting tool smooth. Furthermore, by coating the cutting edge on one side only, the cutting edge becomes self-sharpening. The coating preferably includes at least one layer made of tungsten carbides substantially or entirely free of metallic tungsten.

The present invention relates to self-sharpening knives and othercutting tools having blades provided with a hard laminar or layeredcoating or coatings.

The sharpness of the cutting edge of a knife blade or similar cuttingtool is an important characteristic for both domestic knives andindustrial knives, as well as for cutting tools in general.

It has long been known that the hardness of a blade material is animportant contributor to the ability of a cutting edge of the blade toretain sharpness, as a cutting edge made of softer materials quicklybecomes blunt. On the other hand a knife blade is often made as a thinstrip or as a sheet, and its edge must have some flexibility so as toavoid brittle fracture or chipping when used. The two characteristics ofhardness and flexibility or toughness often contradict with each otheras most hard materials are typically brittle and easy to fracture.

Historically various techniques including quenching, heat treatment oralloying have been used to achieve the best combination of these twocharacteristics.

U.S. Pat. No. 6,105,261 describes a self-sharpening blade having afirst, harder layer with relatively high wear resistance thatsubstantially defines a cutting edge, and a second, softer layer ofmaterial with lower wear resistance, located on one side of the firstlayer. The thickness of the harder layer is between 0.3 microns and 1.5mm. The examples given in this US patent include knife blades producedby metalworking or mechanical processing such as rolling several sheetsof steel, hot pressing and sintering powders containing diamond andother hard materials, as well as coating deposition on plastics. Themechanical processing typically results in the production of arelatively thick layer of hard material, and does not enable a goodblade sharpness to be achieved.

Attempts have been made to produce a knife blade with a hard coating.U.S. Pat. No. 6,109,138 describes a knife blade with one side of itsedge coated with a particulate material in a matrix. It is stated thatthe matrix is softer than the particulate material, and the coating issuch that a considerable number of the particulates project from thematrix thereby defining a cutting tip on the blade edge. This knifeblade has enhanced edge retention characteristics and finds practicalapplications, for example, in domestic kitchen knives. However, kniveswith this type of coating have a number of disadvantages that limittheir applications. The coating process does not allow a thin coating tobe produced—the coating thickness is typically 25-30 microns. Thecoating consists of randomly distributed hard particles in asubstantially softer metal matrix, and this coating structure does nottherefore serve to form a straight self-sharpening edge within thethickness of the coating layer. This sets a limitation on the sharpnessthat can be achieved with a blade having such a thick hard coatinglayer. Furthermore, the cutting edge formed by discrete particles ofhard material projecting from a matrix does not provide a smooth cuttingaction, but instead acts by tensile tearing of the material being cut.This typically requires a higher force to be applied to the cutting edgeas compared to a purely compressive cutting action of a scalpel, forexample. The coatings are normally used in an “as-deposited” condition;in other words, there is no additional or post-machining performed onthe coating itself, which typically has a rough morphology. This surfaceroughness and resulting increased friction between the coating and thematerial being cut further contribute to impede the cutting action.Accordingly, cutting tools provided with this type of coating arerestricted in their application due both to limited sharpness and roughsurface morphology (leading to tearing rather than cutting).

Various attempts to make blades with hard coatings consisting oftungsten carbide particles in cobalt or another soft metal matrix haveshown that a so-called “self-sharpening effect” depends strongly on thecoating structure and properties. For example, the High Velocity OxygenFuel (HVOF) process for deposition of a tungsten carbide in cobaltmatrix coating does provide the self-sharpening effect and is used inpractice. By way of contrast, a similar coating process known as plasmaspraying, when used to deposit a WC/Co coating, does not achieve theself-sharpening effect. Although both HVOF and plasma sprayed coatingsconsist of tungsten carbide particles in a cobalt matrix, and areproduced by similar methods of spraying, the difference in theirperformance to produce cutting tools demonstrates that it is not easy orobvious to achieve the self-sharpening effect. Indeed, producingcoatings that provide the self-sharpening effect depends strongly on thecoating characteristics such as hardness, porosity and microstructure,and requires extensive experimentation and analysis.

EP 0 567 300 describes a hard coating having a columnar crystalstructure that extends away from a surface of a blank and to an outerface of the coating. However, the mechanism of wear and fracture in thecolumnar-structured coating does not provide an optimal structure foredge sharpness. The columnar coating wears by fracture of themicrocrystalline columns and their groups, and does not allow sharpeningwithin the coating layer. As a result, the edge sharpness is defined bythe thickness of the coating.

These techniques, although enhancing the edge retention characteristicsof a blade, do not generally enable a smooth and sharp scalpel-likeblade to be formed. This is particularly important when the blade isused to cut thin paper (such as tissue) and similar materials that caneasily be ripped or tom by an uneven edge.

U.S. Pat. No. 5,799,549 describes razor blades with both sides coatedwith an amorphous diamond coating having a thickness of at least 400angstroms, typically about 2000 angstroms. This coating impartsstiffness and rigidity to a thin blade. However, the coating, which hasa sub-micron thickness (400 angstroms is equal to 0.04 microns, 2000angstroms is equal to 0.2 microns) and is formed on both sides of theblade, does not provide for a self-sharpening effect as the blade isused.

EP 0 386 658 and U.S. Pat. No. 4,945,640 describe a wear-resistantcoating for sharp-edged tools and a method for its production. Thecoating is deposited by the method of chemical vapour deposition (CVD),has thickness from 2 to 5 microns and consists of a mixture of freetungsten with W₂C or W₃C, or a mixture of free tungsten with both W₂Cand W₃C. In all variants of this coating there is an admixture ofrelatively soft metallic tungsten, as a result these coatings typicallyhave moderate hardness, substantially lower than the hardness of puretungsten carbides. Methods of depositing these coatings are furtherdescribed in detail in EP 0 329 085, EP 0 305 917, U.S. Pat. No.4,910,091 and U.S. Pat. No. 5,262,202. The coating is produced from agaseous mixture of tungsten hexafluoride, dimethylether (DME), hydrogenand argon. In this process, low-volatility tungsten oxyfluorides areformed due to the reaction between WF₆ and oxygen-containing DME. Thetungsten oxy-fluorides are difficult to reduce with hydrogen and areburied in the coating layer. This requires an additional heat treatmentof the coating described in U.S. Pat. No. 5,262,202 to improve thecoating characteristics. The coatings described in these publicationshave relatively low hardness (below 3000 Hv, typically 2300 Hv),non-uniform structure, and as a result do not enable self-sharpening tobe achieved. As described in U.S. Pat. No. 4,945,640 and EP 0 386 658,this coating improves the erosion and abrasion resistance of sharp-edgedtools, but does not provide the self-sharpening effect. Without theself-sharpening effect, the hard coating provides only limitedimprovement in the retention of the sharpness of the cutting tool edge.

Coatings that reduce friction between a cutting blade and a materialbeing cut help to improve the cutting action, and to enable a materialto be cut with a lower amount of energy. This has been demonstrated forexample with razor blades coated with thin layer of PTFE, which is knownfor its low friction properties. Although the PTFE coating does notchange the razor blade sharpness, the blade can be moved with lowerforce and thus provides a perception of improved cutting action. SoftPTFE coatings are useful for gentle cutting applications such as shavinghair with a razor blade, but would not survive the more demandingcutting environment faced by machine knives, for example cutting paper,plastics, food products etc. In these conditions, a soft PTFE coatingwill be quickly abraded and worn away. The cutting action of a machineknife would benefit from a durable coating with low friction that isable to resist wear and abrasion.

The surface roughness of a cutting edge bevel, and in particular thesurface roughness of a coating on a cutting edge, also has an effect onthe cutting action. A rougher bevel surface often forms a roughercutting edge with small serrations that contribute to cutting by atensile tearing action. As compared to the purely compressive cuttingaction of a smooth scalpel blade, for example, a rough serrated knifewould require higher force and higher energy for cutting. Serratedknives are considered as longer lasting than knives with a smoothcutting edge, although they have an inferior cutting action, especiallywhen cutting delicate materials.

Embodiments of the present invention seek to provide furtherimprovements in cutting blade construction so as to facilitate cutting,in particular of soft materials that could easily be damaged by tearingor rupture, while maintaining edge retention characteristics of theblade.

Through extensive experimentation and microscopic observations of thewear mechanisms of various coatings, the present applicant hasdiscovered that the best cutting action can be achieved by using anoptimal coating structure and a combination of coating propertiesincluding hardness, thickness and friction coefficient.

Coatings substantially harder than the blade material are found toreduce the wear rate of the cutting blade. When one side of a blade hasthe hard coating, this side will wear significantly less than the otherside having no coating. As the blade is used to cut various materials,micro-wear results in a gradual removal of material from the non-coatedside of the blade.

As a result, after some use the edge is comprised mainly of the hardcoating layer, supported from one side by the base blade material. Atthis stage the behaviour of the coating depends on its microstructure. Acoating consisting of particulate material in a softer matrix will havethe matrix removed by wear, leaving the particulates projecting from thematrix and forming a substantially uneven edge.

A hard coating having a columnar structure will typically fracture alongboundaries between the columnar micro-crystals. When the blade basematerial is removed by wear, leaving the coating edge with insufficientsupport, small micro-crystalline particles will break away from thecoating. In this event, the edge sharpness is defined by the thicknessof the coating layer. A thick coating typically does not providesufficient sharpness, and to improve its cutting ability this type ofblade is often made with serrations/scallops to the non-coated side.This again makes the edge essentially uneven and affects the cuttingaction.

According to a first aspect of the present invention, there is provideda self-sharpening cutting tool having a cutting edge made of a firstmaterial or materials, the cutting edge being coated only on one sidethereof with a coating substantially harder than the first material ormaterials, characterised in that the coating has a layered or laminarmicrostructure aligned substantially parallel to the coated side of thecutting edge.

According to a second aspect of the present invention, there is provideda method of manufacturing a self-sharpening cutting tool, the methodcomprising the steps of

-   -   i) providing a cutting edge made of a first material or        materials;    -   ii) coating only one side of the cutting edge with a coating        substantially harder than the first material or materials;        characterised in that the coating has a layered or laminar        microstructure aligned substantially parallel to the coated side        of the cutting edge.

Coatings having a layered or laminar microstructure exhibit differentbehaviour to the known coatings for cutting edges. When the bladesubstrate material is worn away and does not provide sufficient supportfor all of the coating, micro-particles of the coating break awayfollowing the layered or laminar structure pattern. This leaves athinner coating on the blade edge that enhances the edge sharpness. Thelayered or laminar microstructure also allows sharpening within thethickness of the coating layer, so that an edge sharper than the actualcoating layer can be achieved. The edge is smooth and scalpel-like andmakes a smooth and clear cut, unlike saw-like blades that can tear orrupture the material being cut.

By comparative testing and analysis of cutting tool wear, the presentapplicant has discovered that a layered or laminar structure of coatingwith the hardness of one layer being substantially higher than thehardness of other layers serves to improve the edge sharpness evenfurther. This coating structure enables a bevel or bevels to be formedwithin the coating thickness by known methods of sharpening, and as aresult the edge radius can be reduced to substantially less than thecoating thickness. This structure of the coating further allows acutting edge formed that demonstrates self-sharpening within the coatingthickness. This occurs when the coating layer (or layers) with lowerhardness are worn away first as compared to the hardest layer within thecoating. As a result the hardest layer protrudes from the coating andforms an even sharper cutting edge. Since the coating is continuous andaligned along the tool edge, this cutting edge will be continuous anduniform and will thus provide a smooth cutting action. The continuingwear will maintain the sharpness of the edge formed by this layeredcoating.

The coating may comprise tungsten carbide or mixtures of tungstencarbides substantially or entirely free of metallic tungsten. Bymixtures of tungsten carbides is meant mixtures of two or more of WC,W₂C, W₃C and W₁₂C.

The coating may comprise a multilayered coating, a topmost layer of thecoating comprising tungsten carbide or mixtures of tungsten carbidessubstantially or entirely free of metallic tungsten

The coating may comprise a multilayered coating comprising layers ofdiffering hardnesses, at least one of the layers being a hardest layer.

The coating may comprise a multilayered coating comprising layers ofdiffering hardnesses, a hardest layer of which comprises tungstencarbide or mixtures of tungsten carbides substantially or entirely freeof metallic tungsten.

The hardest layer may be a topmost layer of the coating, or anintermediate layer or a base layer.

The coating may comprise layers of tungsten, tungsten carbides and/ormixtures of tungsten with tungsten carbides alloyed with fluorine inamounts ranging from 0.0005 to 0.5 wt %.

The coating may comprise layers of tungsten and tungsten carbidessubstantially or entirely free of metallic tungsten, the tungstencarbides being alloyed with fluorine in amounts ranging from 0.0005 to0.5 wt %.

The coating may have abase layer of tungsten.

The layers of the multilayer coating may be arranged in sequentiallyincreasing order of hardness from the cutting edge to a topmost layer ofthe coating.

The coating or a topmost layer thereof may have a friction coefficientagainst cemented carbide of 0.3 or less.

The coating may be produced by Chemical Vapour Deposition in a vacuumchamber at a pressure lower than atmospheric pressure and at atemperature above 350° C., preferably from 450° C. to 550° C.

The coating may have a total thickness from 1 to 25 microns, preferably3 to 12 microns.

An exposed surface of the coating may have a roughness Ra of 0.8 micronsor less, preferably 0.5 microns or less.

The coating or a topmost layer thereof may have a microhardness of atleast 2000 kG/mm², preferably at least 2500 kG/mm², and even morepreferably at least 2900 kG/mm².

Experiments made by the present applicant with various coatingthicknesses have shown that to achieve a self-sharpening action, thecoating must be sufficiently thick, and preferably at least 1 to 2 orpossibly 3 microns. On the other hand, coatings thicker than 15-25microns generally do not provide sufficient sharpness. An optimalcoating thickness is therefore within this range of thicknesses

The present applicant has also discovered that coatings having a lowcoefficient of friction and coatings having a smooth surface facilitateblade movement while cutting and further contribute to a smooth cuttingaction and cut quality. This appears to be particularly useful forcutting soft and weak materials like thin paper, which is easily damagedby tearing or rupturing.

The present invention can be applied in relation to various types ofknives or cutting tools, such as for example an ordinary domestic knife,a disk shaped rotary knife used in industry for cutting paper, aguillotine-type knife, and cutting tools of various shapes. Theinvention can be applied in relation to tools for cutting metal, woodand/or plastics (among others), including saws, planes, drills and othermachining tools.

The blade can be made either as a double-bevelled or as asingle-bevelled blade. In the case of a single-bevelled blade, thecoating is formed on a flat or on a bevelled side of the blade, in thecase of a double-bevelled blade, either side of the blade can be coated.

Various coating technologies can be used for deposition of the coatingmaterial, among them Chemical Vapour Deposition (CVD).

CVD tungsten and tungsten carbide coatings described in WO 00/47796 havebeen used by the present applicant to produce a hard coating on a knifeblade. When applied to steel, such coatings generally comprise an innersub-layer usually made of nickel, copper or other metals (preferablyresistant to fluorine), a layer of metal tungsten and further evenharder layers containing tungsten carbide. The coating is produced by aCVD process from a gas mixture containing tungsten hexafluoride (WF₆),hydrogen (H₂) and carbon-containing oxygen-free gas, for example propane(C₃H₈), with the process temperature from 350° C. up to 650° C.,preferably from 400° C. up to 550° C. The use of precursors free fromoxygen, and particularly the innovative thermal pre-activation of thecarbon-containing gas (as described in WO 00/47796) are the advantagesof this method that allow a coating to be formed with very densemicro-crystalline structure and enhanced hardness. The thermalpre-activation of the carbon-containing gas gives effective control ofthe coating composition and serves to produce single-phase tungstencarbides and mixtures thereof, including coating layers consistingsolely of tungsten carbides and which do not contain metallic tungsten,thus providing enhanced hardness. The coating phase composition has beenanalysed by way of X-ray diffraction analysis. Extensive experimentationand analysis of the processes taking place in the CVD furnace show thatthe phase composition of the coating depends principally on thetemperature of the thermal pre-activation, varying for example from 500°C. up to 850° C., the partial pressure of the hydrocarbon gas and thegeneral pressure in the reactor (0.1-150 kPa).

Preliminary activation of the hydrocarbon results in the formation ofthe necessary concentration of hydrocarbon radicals and their associateswith fluorine in the gaseous phase over a wide range. This process, asdescribed in WO 00/47796, makes it possible to alloy the carbides and/ormixtures thereof with fluorine and fluoride-carbon compositions.Fluorine, as the most active chemical element, strengthens theinteratomic bonds when it penetrates into the carbide lattice. It is thestrengthening of the interatomic bonds in the carbide which produces theincrease in hardness.

In addition to the alloying effect, active fluorine and fluoride-carboncompositions form a deposit with a micro-layered, non-columnarstructure, the various layers having different hardnesses due tonon-uniformity of the alloying.

Coatings of thickness from 1 micron up to 25 microns, and hardness from25 GPa up to 40 GPa were applied. The CVD process as described in WO00/47796 makes it possible to produce layered coating structures withhardness varying from one layer to another, this being particularlyfavourable for achieving the self-sharpening effect. The coatingpreferably has a low coefficient of friction, typically below 0.3against cemented carbide (WC/Co). Extensive experimentation was used toidentify the coating parameters to provide the advantages of the presentinvention, including the coating structure, thickness, hardness andfriction coefficient.

The cutting tool of embodiments of the present invention may bemanufactured from one of the following base materials: hard alloys alsoknown as cemented carbide, ceramics such as silicon carbide, siliconnitride, aluminium oxide, zirconium oxide, carbon-carbon compositionmaterials etc., various iron-containing alloys such as iron, carbonsteels, stainless steels, tool and high-speed steels and cast iron, orother materials from the following list: copper, silver, gold, cobalt,nickel, silicon, tantalum, niobium, vanadium, tungsten, molybdenum,carbon, boron, their alloys, compounds and mixtures, and also titaniumalloys.

If the cutting tool is made of a chemically active base material such asiron, carbon steels, stainless steels, tool and high-speed steels, castiron, titanium alloys etc., it is preferable to deposit intermediatecoatings containing materials chemically resistant to hydrogen fluoride,such as those from the following list: copper, silver, gold, cobalt,nickel, rhodium, rhenium, platinum, iridium, tantalum, molybdenum,niobium, vanadium and boron. An intermediate coating of thickness 0.1-15microns, preferably 0.5-5 microns, may be deposited by electrochemicalor chemical deposition from aqueous solutions, melt electrolysis,chemical or physical vapour deposition (e.g. by means of magnetronspraying) or by other methods.

The cutting tool with an intermediate coating, preferably of nickel,copper or boron, is placed into a CVD reactor furnace, and has aninternal layer of tungsten deposited first, followed by deposition of acoating consisting mainly of tungsten carbides or their mixtures, ormixtures of tungsten with carbon. The total thickness of the CVD coatingmay be from 1 micron up to 25 microns, with the ratio of the thicknessesof the internal and external layers ranging from 1:1 to 1:600.

After depositing the coating on one side of a cutting edge, the otherside may be additionally sharpened by grinding or by any othertechnique. This forms an edge consisting of the hard coating layer andthe basic blade material, usually steel. In the course of use, the basicblade material is removed by wear and abrasion, leaving a thin layer ofthe hard coating. When the basic blade material is not sufficient tosupport the hard coating, microscopic pieces of the coating may bebroken away, typically following a layered pattern of the coatingstructure. This gives a smooth scalpel-like blade edge with sharpnessthat can not be achieved with thicker coatings or coatings having acolumnar structure. Use of the blade actually enhances the edgesharpness.

Alternatively or in addition, in order to improve the cutting actionfurther, the coated side of the blade can be additionally polished orground so as to remove roughness on the coated surface that wouldotherwise brush against material being cut. This additional polishing orgrinding could be made in a direction along the cutting edge, so thatthe polished blade will move smoothly and thus reduce the force requiredfor cutting.

A smooth scalpel-like blade edge with enhanced sharpness made using thepresent invention is a particular advantage when the blade is cuttingsoft materials that could be easily damaged, for example thin toilettissue. It has also been found by the present applicant that to achievethis scalpel-like blade, a sharpening process should include a stage ofdressing or sharpening with a sharpening tool moving along the cuttingedge to remove burrs and projecting areas of the coating or substratematerial.

For a better understanding of the present invention and to show how itmay be carried into effect, reference shall now be made, by way ofreference, to the following examples and the accompanying drawing, inwhich:

FIG. 1 shows a graph indicating the cutting performance of knives withvarious coatings.

EXAMPLES

The examples given below illustrate the invention specifically inrelation to the use of CVD coatings. However, these examples are not tobe taken as limiting the scope of the invention to those specificprocesses, since other processes may also have the properties required.

Example 1

A series of test blades were made from martensitic stainless steelhaving the following specification:

0.35% carbon

12.5% chromium

Hardness: 54 Re

Dimensions: 120 mm×25 mm with a primary edge angle of 15°/side,

The blades were subsequently coated by Hardide® on one side only asfollows: Coating hardness/ Blade Coating Thickness/μm kG/mm² 5 & 6Hardide ® H (hard) 5 3100 7 & 8 Hardide ® H (hard) 10 3100 9 & 10Hardide ® M (multi-layer) 12 2100

Hardide®-H coating has a sub-layer of metal tungsten 0.5 microns thick,and a layer of W₂C 5 or 10 microns thick. Hardide®-M has a sub-layer ofmetal tungsten 0.5 microns thick, a layer of metal tungsten with carbon10 microns thick, and a hardest top layer of W₂C 2 microns thick.

The blades were then honed on a 320 grit oil stone on the non-coatedside at an angle of 20° using an accurate blade honing fixture.

Evaluation Method:

The blades were subjected to cutting tests for sharpness and life to ISO8442.5. The blades were mounted in the ISO cutting test machinespecified such that the blades cut through 10 mm wide strips of manilacard. The card comprised 95% cellulose fibre, the balance being made upof silica. The effect of the latter is to increase the wear rate duringcutting. The blade was cycled back and forth over a distance of 40 mm ata speed of 50 mm/second under a load of 50N. The amount of card cut percycle was recorded, this being a measure of the blade sharpness. All theblades were initially subjected to 60 cycles.

After the initial 60 cycles, the results were examined, and blades 5 and7 were then subjected to further cycles to a total of 1060 cycles.

The results are shown in summary in Table 1 and FIG. 1. For comparison,typical figures for 15/20 micron Co/WC sprayed coatings and a standardmartensitic stainless steel blade are also shown. TABLE 1 Initial 60cycle Total at Blade sharpness sharpness Total at 60 1060 5 (5 μm H) 1220.9 1212 16897 7 (10 μm H) 26.3 27.9 1603 22233 9 (12 μm M) 31.4 26.71715 — Co/WC 18 17 1050 15000 Standard martensitic 40 3 647 — steelblade

The life performance of the various blades indicate that both Hardide®-Hcoatings and multilayer coatings perform well in terms of edgeretention. The hard coatings generate a self sharpening action of theblades, as their sharpness increases with the number of cuts: the curvesb5 and b7 of the attached FIG. 1 show a growth in cut depth with thenumber of cuts.

The coating becomes exposed at the tip as the base metal side of theedge is worn away, gradually making the blade sharper. However, after acertain time the coating partially collapses leaving a fractured andblunt tip or edge. The self-sharpening procedure then repeats itselfover a significant number of cycles.

Comparison of these results with other coatings, such as for exampletypical WC/Co sprayed coating, shows that Hardide® coatings performbetter in terms of sharpness. Well-known kitchen knives with a tungstencarbide/cobalt coating of thickness around 18/20 μm only achieve a firstcut sharpness of around 18-20 mm and remain at approximately the samelevel, whereas the knives with Hardide® coating managed to achieve muchbetter initial sharpness, this being maintained at a higher level overthe testing period.

Example 2

A set of nine domestic knives made of stainless steel were coated withCVD coating, consisting of layers of nickel, tungsten and tungstencarbide. The knives were positioned in a vacuum chamber so that one sideof each knife was masked. The coating was produced with three differentthicknesses: three blades with a coating 6 microns thick, three bladeswith a coating 9 microns thick, and three blades with a coating 13microns thick. The other side of the blade was sharpened by grinding,including edge dressing by way of a sharpening tool being moved alongthe cutting edge so as to remove burrs and projecting parts of coatingand steel. The coating produced had a friction coefficient against WC/Coof 0.2.

The knives were tested on a test rig to cut 50 mm thick cardboard blocksunder a fixed load by reciprocating movements, while the number ofstrokes required to cut the block was counted.

All the tested knives cut the first block of cardboard in approximately5 strokes, and this number of strokes was generally maintained andgradually reduced during the tests which involved cutting 100 blocks.For comparison, a standard non-coated sharp knife cut the first block in2-3 strokes, but the number of strokes increased up to 70-100 when itcut the 5th block as a result of the edge becoming blunt very quickly.

This test demonstrated that the coating provides a self-sharpeningcutting edge.

Example 3

Two disk rotary knives were coated with a CVD coating consisting oflayers of nickel, tungsten and tungsten carbide. The disks werepositioned in a vacuum chamber so that one side of the knife had acoating of 5 microns in one case and 10 microns in the other. Thecoating micro-hardness was 3700 Hv. The disk knives were sharpened bydressing the edge and grinding another side of the edge, and observationof the edge under a microscope showed it to be smooth with a sharpscalpel-like edge.

Tests of the knives showed enhanced cutting action without damage to thematerial being cut (tissue paper). The standard blades were normallyreplaced every 12 hours for re-sharpening The Hardide®-coated bladeswere used continuously for 10 weeks without re-sharpening, and theirsharpness and facilitated cutting action remained suitable for thisdemanding application. The increase in the continuous operation of theblade was in excess of factor of 100, this being due to the bladeself-sharpening effect.

Example 4

Cutting tools for cutting polyethylene film from a solid block ofplastic (polyethylene) by a process known as skiving were made of toolsteel as a long bar with one or two corners profiled to make a sharpcutting edge. Because of the abrasive nature of the material and theprocess the tool had to be replaced for re-sharpening several times aday, and the longest any blade could last in production was around oneday. Apart from quickly becoming blunt, traditional blades werevulnerable to chipping of the cutting edge caused by contamination inthe plastic block.

Four cutting tools were coated with CVD tungsten carbide on a flat sideof the bar, with the coating thickness varying from 1.5 microns up to 8microns. The coating had micro-hardness of 3600 Hv and a frictioncoefficient against WC/Co of 0.2.

The trials continued for 7 months, and during the whole period of thetrials the Hardide®-coated tools were re-sharpened only once, and noother cutting tools were used. The tests showed that the tool sharpnesswas maintained over a period at least 70 times longer than normal toollife.

Example 5

A blade of a plane was coated with CVD tungsten carbide so that a flatside of the blade had a hard coating. The other, bevelled, side of theblade was sharpened

The plane maintained its blade sharpness for an operational period up to3 times longer than a period after which a standard blade requiressharpening, before testing was stopped. It is expected that the coatedblade will maintain its sharpness for even longer periods of operation.

Example 6

A metal cutting tool made of tool steel was coated with multi-layer CVDtungsten carbide on one side of its cutting edge. The coating consistedof a 1 micron nickel sub-layer, a 0.5 micron tungsten layer followed byalternating layers of tungsten carbide approximately 1.5 micron thickand tungsten approximately 0.5 micron thick, up to a total thickness of10 microns. The coating had a friction coefficient against WC/Co of 0.2.

The tool was used for cutting aluminium, and tests showed that the toolhad enhanced cutting quality, reduced sticking of aluminium shavings,and the tool remained sharp for at least four times longer than a normalnon-coated tool. The tests were then stopped, but it is expected thatthe coated tool could continue cutting aluminium while remaining sharpdue to the coating, thereby ensuring an enhanced quality of cut.

The preferred features of the invention are applicable to all aspects ofthe invention and may be used in any possible combination.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, mean “including but not limited to”, andare not intended to (and do not) exclude other components, integers,moieties, additives or steps.

1. A self-sharpening cutting tool comprising: a cutting edge made of afirst material, the cutting edge being coated only on one side thereofwith a coating substantially harder than the first material, wherein thecoating has a layered or laminar microstructure aligned substantiallyparallel to the cutting edge of the coated side.
 2. A tool as claimed inclaim 1, wherein the coating comprises tungsten carbide or mixturesthereof, substantially or entirely free of metallic tungsten.
 3. A toolas claimed in claim 1, wherein the coating is a multilayered coating,with a topmost layer of the coating comprising tungsten carbide ormixtures thereof, substantially or entirely free of metallic tungsten.4. A tool as claimed in claim 1, wherein the coating is a multilayeredcoating comprising layers of varying hardness.
 5. A tool as claimed inclaim 1, wherein the coating is a multilayered coating comprising layersof varying hardness, the hardest layer of which comprises tungstencarbide or mixtures thereof substantially or entirely free of metallictungsten.
 6. A tool as claimed in claim 4, wherein the hardest layer isa topmost layer of the coating.
 7. A tool as claimed in claim 4, whereinthe hardest layer is an intermediate layer of the coating
 8. A tool asclaimed in claim 4, wherein the hardest layer is a base layer of thecoating.
 9. A tool as claimed in claim 4, wherein the coating compriseslayers of tungsten, tungsten carbides and/or mixtures of tungsten withtungsten carbides alloyed with fluorine in amounts ranging from about0.0005to about 0.5 wt %.
 10. A tool as claimed in claim 4, wherein thecoating comprises layers of tungsten and tungsten carbides substantiallyor entirely free of metallic tungsten, being alloyed with fluorine inamounts ranging from about 0.0005 to about 0.5 wt %.
 11. A tool asclaimed in claim 4, wherein the coating has a base layer of tungsten.12. A tool as claimed in claim 3, wherein the layers are arranged insequentially increasing hardness from the cutting edge to a topmostlayer of the coating.
 13. A tool as claimed in claim 1, wherein thecoating or a topmost layer thereof has a friction coefficient againstWC/Co of no more than 0.3.
 14. A tool as claimed in claim 1, wherein thecoating is produced by Vapour Deposition in a vacuum chamber at apressure lower than atmospheric pressure and at a temperature aboveabout 350° C.
 15. A tool as claimed in claim 1, wherein the coating hasa total thickness from about 1 to about 25 micrometers.
 16. A tool asclaimed in claim 1, wherein an exposed surface of the coating has aroughness of no more than about 0.8 Ra micrometers.
 17. A tool asclaimed in claim 1, wherein the coating or a topmost layer thereof has amicrohardness of at least about 2000 kG/mm².
 18. A tool as claimed inclaim 1, wherein an exposed surface of the coating is ground or polishedin a direction substantially parallel to the coated surface of thecutting edge.
 19. A method of manufacturing a self-sharpening cuttingtool, the method comprising: providing a cutting edge made of a firstmaterial or materials; and coating only one side of the cutting edgewith a coating substantially harder than the first material ormaterials; wherein the coating has a layered or laminar microstructurealigned substantially parallel to the coated side of the cutting edge.20. A method according to claim 19, wherein the coating comprisestungsten carbide or mixtures thereof, substantially or entirely free ofmetallic tungsten.
 21. A method according to claim 19, wherein thecoating is a multilayered coating, the topmost layer of the coatingcomprising tungsten carbide or mixtures thereof, substantially orentirely free of metallic tungsten.
 22. A method according to claim 19,wherein the coating is a multilayered coating comprising layers ofvarying hardness.
 23. A method according to claim 19, wherein thecoating is a multilayered coating comprising layers of varying hardness,the hardest layer of which comprises tungsten carbide or mixturesthereof, substantially or entirely free of metallic tungsten.
 24. Amethod according to claim 22, wherein the hardest layer is a topmostlayer of the coating.
 25. A method according to claim 22, wherein thehardest layer is an intermediate layer of the coating.
 26. A methodaccording to claim 22, wherein the hardest layer is a base layer of thecoating.
 27. A method according to claim 22, wherein the coatingcomprises layers of tungsten, tungsten carbides or mixtures thereof,alloyed with fluorine in amounts ranging from about 0.0005 to about 0.5wt %.
 28. A method according to claim 22, wherein the coating compriseslayers of tungsten and tungsten carbides substantially or entirely freeof metallic tungsten, being alloyed with fluorine in amounts rangingfrom about 0.0005 to about 0.5 wt %.
 29. A method according to claim 22,wherein the coating has a base layer of tungsten.
 30. A method accordingto claim 21, wherein the layers are arranged in sequentially increasingorder of hardness from the cutting edge to a topmost layer of thecoating.
 31. A method according to claim 19, wherein the coating or atopmost layer thereof has a friction coefficient against WC/Co of nomore than 0.3.
 32. A method according to claim 19, wherein the coatingis applied by Chemical Vapour Deposition in a vacuum chamber at apressure lower than atmospheric pressure and at a temperature of no lessthan about 350° C.
 33. A method according to claim 19, wherein thecoating is applied to a total thickness from about 1 to about 25micrometers.
 34. A method according to claim 19, wherein an exposedsurface of the coating after application has a roughness Ra of no morethan about 0.8 micrometer.
 35. A method according to claim 19, whereinthe coating or a topmost layer thereof has a microhardness of at leastabout 2000 kG/mm².
 36. A method according to claim 19, wherein anexposed surface of the coating, after application of the coating, isground or polished in a direction substantially parallel to the coatedsurface of the cutting edge.
 37. A tool as claimed in claim 1, whereinthe coating is produced by Chemical Vapour Deposition in a vacuumchamber at a pressure lower than atmospheric pressure and at atemperature from about 450 to about 550° C.
 38. A tool as claimed inclaim 1, wherein the coating has a total thickness of about 3 to about12 micrometers.
 39. A tool as claimed in claim 1, wherein an exposedsurface of the coating has a roughness Ra of about 0.5 microns or less.40. A tool as claimed in claim 1, wherein the coating or a topmost layerthereof has a microhardness of at least 2500 kG/mm².
 41. A tool asclaimed in claim 1, wherein the coating or a topmost layer thereof has amicrohardness of at least 2900 kG/mm².
 42. A method according to claim19, wherein the coating is applied by Chemical Vapour Deposition in avacuum chamber at a pressure lower than atmospheric pressure and at atemperature from about 450 to about 550° C.
 43. A method according toclaim 19, wherein the coating is applied to a total thickness from about3 to about 12 micrometers.
 44. A method according to claim 19, whereinan exposed surface of the coating after application has a roughness Raof no more than 0.5 micrometers.
 45. A method according to claim 19,wherein the coating or a topmost layer thereof has a microhardness of atleast 2500 kG/mm².
 46. A method according to claim 19, wherein thecoating or a topmost layer thereof has a microhardness of at least 2900kG/mm².